The alpha1-adrenergic receptors (alpha1-ARs) play a vital role in the regulation of systemic arterial blood pressure, blood flow, cardiac contraction and as mediators of growth responses. Three receptors, the alpha1A, the alpha1B and the alpha1D-ARs have been isolated, cloned and characterized. This is an application to continue studies designed to increase our understanding of the cardiovascular regulatory biology of the alpha1-ARs. We will focus our studies on the alpha1B and alpha1D-ARs. The alpha1B-AR has properties typical of a G-protein coupled receptor (GPCR). In contrast, the alpha1D-AR has many characteristics that are atypical for a GPCR. Specific Aim # 1 builds on observations made by us and others that there are significant differences in the basal cellular localization, agonist mediated receptor internalization and desensitization between the alpha1b and alpha1D-ARs. We will test the hypothesis that these differences are due to sequence diversity between these receptors in key subdomains, particularly the C-terminus. Specific Aims # 2 and 3 will focus on the regulatory activities of the alpha1B and alpha1D ARs and will utilize vascular smooth muscle cells, cardiomyocytes, in vitro responses in isolated blood vessels, the isolated heart and in vivo measurements. We will also use newly developed lines of transgenic mice in assessing these hypotheses. Aim # 2 will focus on the alpha1D-AR and evaluate the hypothesis that this receptor is a specific regulator of vascular smooth muscle contraction with minimal effects on cardiomyocytes. In Aim # 3 we will test the hypothesis that the alpha1B-AR plays an important role in mediating growth responses and as a regulator of contractile function in the heart. We will show that the alpha1B-AR has limited involvement in the contraction of vascular smooth muscle. We have proposed a comprehensive experimental design that applies contemporary molecular biologic, genetic, cellular and immunologic approaches in combination with in vitro and in vivo assessment of physiologic function. Specific hypotheses will be evaluated in a fashion that will yield definitive mechanistic conclusions. Considering the health-related risks of hypertension and heart failure, a better understanding of the role of the alpha1-ARs in these pathophysiologic conditions is highly relevant. Knowledge of the role of these receptors in these disorders can also aid in the development of newer and safer drug therapies.